In the UMTS (Universal Mobile Telecommunications System) network, the specifications of long term evolution (LTE) have been drafted for the purpose of further increasing high speed data rates, providing lower delays and so on (see non-patent literature 1). Also, successor systems of LTE (also referred to as, for example, “LTE-advanced” or “LTE enhancement” (hereinafter referred to as “LTE-A”)) have been developed for the purpose of achieving further broadbandization and increased speed beyond LTE.
Furthermore, in relationship to future radio communication systems (Rel. 13 and later versions), a system (“LTE-U” (LTE Unlicensed)) to run an LTE system not only in frequency bands that are licensed to communications providers (operators) (licensed bands), but also in frequency bands that do not require license (unlicensed bands), is under study.
While a licensed band refers to a band in which a specific operator is allowed exclusive use, an unlicensed band (also referred to as a “non-licensed band”) refers to a band which is not limited to a specific operator and in which radio stations can be provided. For unlicensed bands, for example, the 2.4 GHz band and the 5 GHz band where Wi-Fi and Bluetooth (registered trademark) can be used, and the 60 GHz band where millimeter-wave radars can be used are under study for use.
In LTE-U operation, a mode that is premised upon coordination with licensed band LTE is referred to as “LAA” (Licensed-Assisted Access) or “LAA-LTE.” Note that systems that run LTE/LTE-A in unlicensed bands may be collectively referred to as “LAA,” “LTE-U,” “U-LTE,” and so on.
For unlicensed bands in which LAA is run, a study is in progress to introduce interference control functionality in order to allow co-presence with other operators' LTE, Wi-Fi or different systems. In Wi-Fi, LBT (Listen Before Talk), which is based on CCA (Clear Channel Assessment), is used as an interference control function in the same frequency. In Japan and Europe, the LBT function is stipulated as mandatory in systems such as Wi-FI that is run in the 5 GHz unlicensed band.